Before rebooting the TiVo to fix this, try a couple of things first..
1: Check the front-left of your TiVo for any flashing green LEDs, if the heartbeat light is flashing your box has no internet connection so anything using online will fail - TV Guide included. Trying to use OnDemand is another good test.
2: If this is the case, then there's the problem and further tests are a waste of time. If the light is on, then follow Home > Help & Settings > Settings > Network > Connect to VM services now.
Wait 30 minutes or so, and check your TV Guide.
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